Electronic circuits (hereinafter referred to as “in-vehicle electronic circuits”) obtained by soldering electronic parts to printed circuit boards are mounted on a vehicle. Such in-vehicle electronic circuits are used in units for electrically controlling components such as engine, power steering and brake, and are safety parts which are very important for the vehicle travel. Therefore, the in-vehicle electronic circuits must be operable in a stable state over a prolonged period of time without causing any failure. In particular, an in-vehicle electronic circuit for engine control is often disposed in the vicinity of the engine and is in a rather severe operating environment.
In fact, when the engine is rotated, the vicinity of the engine where such an in-vehicle electronic circuit is disposed has a high temperature of 100° C. or more. On the other hand, when the engine rotation is stopped, the outside air temperature in the vicinity of the engine in the winter season lowers to −30° C. or less in cold regions such as North America and Siberia. Therefore, the in-vehicle electronic circuit is exposed to heat cycles in a range from −30° C. or less to +100° C. or more by repeatedly operating and stopping the engine.
When an in-vehicle electronic circuit is disposed for a long period of time in an environment in which the temperature thus varies considerably, this causes thermal expansion and contraction of electronic parts and a printed circuit board. However, since there is a large difference between the coefficient of linear expansion of each electronic part and that of the printed circuit board, a certain degree of thermal displacement is repeatedly applied to each of portions where the electronic parts are joined to the printed circuit board by soldering (hereinafter referred to as “solder joints” as appropriate) during the use under the foregoing environment. The solder joints are then stressed and are finally broken.
Accordingly, a solder alloy joining the electronic parts to the printed circuit board is required to be expandable and contractible, in other words, to have ductility so as to prevent fracture of the solder joints. The solder alloy having excellent ductility reduces the stress caused by the thermal displacement as described above.
A vehicle not only runs on a flat road but may also run on a rugged road. Therefore, the vehicle is subject to vibration and impact from the road surface and in-vehicle electronic circuits mounted on the vehicle are also subject to such vibration and impact. Then, since solder joints of the in-vehicle electronic circuits need to have a sufficient strength to withstand such vibration and impact, the solder alloy itself also needs to have a higher tensile strength.
Since solder alloys for in-vehicle electronic circuits are thus required to have severe characteristics, few propositions have heretofore been made therefor.
From the viewpoint of heat cycle characteristics, Patent Literature 1 discloses an Sn—Ag—In—Bi solder alloy for use in a general electronic device to which Sb and Ni are added, the solder alloy comprising: 0.5 to 5% of Ag; 0.5 to 20% of In; 0.1 to 3% of Bi; in total up to 3% of at least one of Sb, Zn, Ni, Ga and Cu; and a balance of Sn. A solder alloy whose composition is closest to that in the invention to be described later and is specifically disclosed is an Sn-3.5Ag-12In-0.5Bi-0.2Sb-0.3Ni solder alloy described in Example 22 of Patent Literature 1.